The present invention relates to a miniature high frequency (h-f) coil assembly or transformer of the kind suitable for use with a hybrid integrated circuit, more particularly to a miniature h-f coil assemb1y or transformer of the kind which has both a high inductance and a high Q and which has a variable inductance, and still more particularly to a method of fabricating the above coil assembly or transformer.
A h-f coil or transformer, which is used in an intermediate frequency (i-f) stage of electronics appliances such as a radio or television receiver, is required to have a considerable number of windings and to have a variable inductance. These requirements arise from the facts that (1) the inductance and the Q of a coil should have high values in the i-f range, and (2) the coil should compensate for fluctuations in circuit operation and variation between individual parts and also between fully assembled h-f coils.
Prior to a description of the present invention, reference will be made to two conventional h-f coil assemblies used in an i-f stage in connection with FIGS. 1 and 2 of the accompanying drawings. The principle of the prior art of FIG. 1 has been disclosed in U.S. Pat. Nos. 3,278,877 and 3,458,844, while the prior art of FIG. 2 has been disclosed in Japanese Utility Model application published on May 19, 1972 under the number of 47-13805. In FIG. 1, one of the prior art is schematically illustrated in an elevational cross section. A bobbin 5 is snugly received, at its lower portion, in a through bore 2 of a substrate or base 1, carrying a coil 7 between its flanges 3. The bobbin 5 is made of a dielectric or electrically insulative material such as synthetic resin, and is formed with a through bore 9. The coil 7 is electrically connected to electrodes 4 at its terminal lead wires (not shown), respectively. A threaded core or slug 11 is movably received in the through bore 9 and as a consequence the inductance of the coil 7 can be varied by turning or screwing the core 11 at either of two slots (no numerals) formed in the opposite ends of the core 11. An external magnetic core 13 is snugly mounted on the bobbin 5 in a manner to cover the same, shielding magnetic fluxes generated. The h-f coil assembly as referred to above has a sufficient space between the flanges 3, so that a considerable number of windings can be accommodated therebetween. Therefore, such a coil assembly is suitable for use in an i-f stage, in that the coil can have a high inductance and Q and at the same time a variable inductance in h-f range.
However, this coil assembly encounters a problem that it can not be miniaturized to the degree where it can be used with an integrated circuit. Furthermore, the coil assembly just described consists of many parts, and resultantly it is very difficult to assemble where the parts used in the construction thereof are very small. Hence it will thus be understood that the assembly problem is inherent since the number of parts cannot be reduced with this design. The dimensions of the coil assemly of FIG. 1 are, for example, about 10 mm both in width and in depth, and about 18 mm in height. These dimensions are too large compared with those of other parts used in a hybrid integrated circuit.
In assembling the coil element of FIG. 1, the base 1 and the hollow bobbin 5 are preformed, then, the bobbin 5 is mounted on the base 1 to which the electrodes 4 are attached. The coil 7 is wound between the flanges 3, being connected to the electrodes 4 at its lead wires, respectively. The movable core 11 is inserted into the through bore 9. Finally, the external magnetic core 13 is fixedly deposited on the semi-assembled structure.
When such a coil assembly is adapted to be used with the hybrid integrated circuit, the base 1 and the bobbin 5 should be reduced in size. In this case, however, various difficulties arise that (1) mass production tolerances when making each of the parts are difficult to hold within sufficient limits whereby manufacturing costs increase, and (2) assembly processes become complicated.
In FIG. 2, the other prior art is schematically illustrated in perspective. A substantially channel-shaped magnetic core member 15 carries a coil 19 around its center portion as shown. The coil 19 is electrically connected to two electrodes 21 which are formed on both ends of the core member 15 and which are insulated from said core member 15 by a suitable insulative film (not shown). This type of h-f coil element is of very simple configuration and as a consequence can be manufactured with ease, and, furthermore, can be reduced in size to such an extent as to be used together with a hybrid integrated circuit. However, this type of h-f coil element encounters problems that (1) the magnetic shielding is not easily provided, (2) the inductance is not variable, and (3) a high inductance and a high Q can not be obtained at an i-f range.